FIG. 3 illustrates a single-row deep-groove ball bearing 1 that is used as a rolling bearing that is assembled in a rotation support section of various kinds of mechanical devices. The ball bearing 1 has: an inner ring 3 having an inner-ring raceway 2 formed around the outer-circumferential surface thereof; an outer ring 5 having an outer-ring raceway 4 formed around the inner-circumferential surface thereof, plural balls 6 provided between the inner-ring raceway 2 and the outer-ring raceway 4; and a cage 7 holding the balls 6 so as to be able to roll freely. Furthermore, in the example illustrated in the figure, openings on both ends of a space 9 where the balls 6 and cage 7 are located, are closed off by a pair of seal rings 8.
There are various kinds of cages, however, the cage 7 illustrated in the figure is called a ribbon cage, with the construction thereof being disclosed, for example, in JP H07-301242 (A), JP H10-281163 (A), JP H11-179475 (A), JP 2009-008164 (A), JP 2009-236227 (A) and the like. As illustrated in FIG. 4, the ribbon cage 7 is constructed such that a pair of cage elements 10 is joined by plural rivets 11. Each of the cage elements 10 is made of a metal plate material such as steel plate, stainless steel plate and the like, and obtained by bending the material in the circumferential direction while at the same time punching out the material into a ring shape by a pressing process so that there are flat sections 12 and semi-circular shaped sections 13 that are alternatively arranged in the circumferential direction. The pair of cage elements 10 is such that, with the flat sections 12 placed face to face with each other, the cage elements are joined and fastened together by rivets 11, and the portions that are surrounded by the semi-circular shaped sections 13 of one of the cage elements 10 and the semi-circular shaped sections 13 of the other cage element 10 function as pockets 14 for holding the balls 6 so as to roll freely.
In order to maintain the product quality of this kind of ribbon cage 7 and display the desired performance for the ball bearing 1, it is essential that the characteristics of the joint sections between the flat sections 12 made using the rivets 11 be good. For example, the precision of the dimensions of each of the rivets 11 has an effect on the characteristics of the joint sections between the flat sections 12. However, from the aspect of product cost, it is desirable that the rivets 11 be made at a very low cost, and thus strictly restricting the manufacturing tolerance is not preferred. Therefore, some volume variations occur among plural rivets 11. The problems that occur due to these volume variations will be explained with reference to FIGS. 5A and 5B.
As illustrated in FIGS. 5A and 5B, in order to join and fasten the pair of cage elements 10 using rivets 11, circular holes 15 are formed in each of the portions where the flat sections 12a, 12b of the cage elements 10 come together. Each of the rivets 11 is made of a plastically deformable metallic material such as mild steel, copper alloy or the like that is capable of maintaining the required strength and rigidity, and has an outward-facing flange shaped head section 16 and a circular column shaped rod section 17. The outer diameter of the head sections 16 is larger than the inner diameter of the circular holes 15, and the outer diameter of the rod sections 17 in the state before the tip-end sections are crimped is the same as or a little smaller than the inner diameter of the circular holes 15.
In order to join the flat sections 12a, 12b together with rivets 11, the rod sections 17 are inserted through the circular holes 15, while at the same time the inside surfaces of the head sections 16 that face one of the flat sections 12a are brought into contact with the outside surfaces of the flat sections 12a that are the surfaces on the opposites side of the other flat sections 12b. In this state, the tip-end sections of the rod sections 17 protrude from the outside surfaces of the other flat section 12b. Furthermore, the rivets 11 are pressed and crushed in the axial direction by a pair of crimping dies 18a, 18b, to form crimped sections 19 on the tip-end sections of the rod sections 17. Of the crimping dies 18a, 18b, truncated cone shaped concave sections 20a, 20b are formed in the portions that are aligned with the circular holes 15 and that press and crush the rivets 11 from both sides in the axial direction. The pair of cage elements 10 are joined and fastened together by rivets 11 at plural locations that are uniformly spaced in the circumferential direction, and when doing this, the plural rivets 11 are crimped simultaneously. In other words, the plural rivets 11 are plastically deformed simultaneously by the pair of crimping dies 18a, 18b that have ring-shaped pressure sections (crimping sections) located on both sides of the pair of cage elements 10. By regulating the amount that the crimping dies 18a, 18b move (approach to each other), the dimension in the axial direction of all the rivets 11 coincide after crimping has been completed. That is, after the crimping dies 18a, 18b arrive at the position in the stroke nearest each other, it is determined that crimping of the rivets 11 is completed.
As long as the volume of the plural rivets 11 is an appropriate value, and as long as the volume of each rivet 11 is the same, then at the same time that the head sections 16 are filled into the concave sections 20a of one of the crimping dies 18a and plastically deformed a little, the tip-end sections of the rod sections 17 are filled into the concave sections 20b of the other crimping die 18b and plastically deformed a lot, and the inside surfaces of the flat sections 12a, 12b that face each other come in contact with each other with no space in between. In this case, the characteristics of the joint sections with the rivets 11 between the pair of cage elements 10 are good, and no particular problems occur.
However, when the volume of the rivet 11 is too small, then, as illustrated in FIG. 5A, even when the crimping dies 18a, 18b arrive at the position in the stroke nearest each other, the tip-end section of the rod section 17 of the rivet 11 is not sufficiently filled inside the concave section 20b of the other crimping die 18b, so the shape and size of the crimped section 19 that is formed from the tip-end section are not sufficient. In this state, the flat sections 12a, 12b located between the crimped section 19 and the head section 16 cannot be sufficiently pressed together, so a gap 21 occur between the flat sections 12a, 12b. When the ball bearing 1 is operated in a state when such a gap 21 occurs, it becomes easy for strange noise such as a beating noise or vibration to occur. Moreover, not only is the strength and rigidity of the obtained cage 7 insufficient, in severe cases, the inner diameter of the pockets 14 becomes larger than the proper value, and there is a possibility that balls 6 will drop out from the pockets 14.
On the other hand, as illustrated in FIG. 5B, when the volume of a rivet 11 is excessively large, the outer circumferential edge part of the head section 16 protrudes out into a gap between the inside surface of one of the crimping dies 18a and the outside surfaces of one of the flat sections 12a, and a thin burr 22 is formed in that portion. There is also a possibility that this kind of burr will be formed in the outer circumferential edge part of the crimped section 19. The thin burr 22 can easily break away from the outer circumferential edge of the head section 16 or crimped section 19, and a burr 22 that has broken away becomes a metal fragment that can become caught inside the space 9 of the ball bearing 1; and as the ball bearing 1 rotates, there is a possibility that the inner-ring raceway 2, outer-ring raceway 4 and rolling surfaces of the balls 6 (see FIG. 3) will become damaged, and that the durability of the ball bearing 1 will be impaired.
These kinds of problems can be suppressed by sufficiently reducing any dispersion in the volume of the rivets 11 however, doing so also causes an increase in the manufacturing cost of the rivets 11. Plural rivets 11 are used for one cage 7, so an increase in the manufacturing cost of the rivets 11 has a large effect on the manufacturing cost of a ball bearing 1 that includes the cage 7.
All of the rivets 11 assembled in the cage 7 are simultaneously crimped by the pair of crimping dies 18a, 18b, so the amount of compression for each rivet 11 cannot be adjusted according to differences in volumes of the rivets 11. Crimping each of the rivets 11 is not industrially performed and is not realistic from the aspect of balancing the strength of each of the joints.
Technology is disclosed in JP H11-179475 (A) in which the characteristics of rivet joint sections between flat sections are improved by devising the shape of the tip-end section of the rod section of the rivets and the shape of the crimping dies for pressing and crushing the tip-end section of the rod section. Moreover, JP 2009-008164 (A) discloses technology that suppresses the residual stress that occurs in the rivets due to the crimping process. However, in these conventional arts, eliminating problems due to differences in the volume of the plural rivets is not particularly taken into consideration.